Stents are generally cylindrical shaped devices that are radially expandable to hold open a segment of a blood vessel or other anatomical lumen after implantation into the body lumen. Stents have been developed with coatings to deliver drugs or other therapeutic agents.
Stents are used in conjunction with balloon catheters in a variety of medical therapeutic applications including intravascular angioplasty. For example, a balloon catheter device is inflated during PTCA (percutaneous transluminal coronary angioplasty) to dilate a stenotic blood vessel. The stenosis may be the result of a lesion such as a plaque or thrombus. After inflation, the pressurized balloon exerts a compressive force on the lesion thereby increasing the inner diameter of the affected vessel. The increased interior vessel diameter facilitates improved blood flow. Soon after the procedure, however, a significant proportion of treated vessels re-narrow.
To prevent restenosis, short flexible cylinders, or stents, constructed of metal or various polymers are implanted within the vessel to maintain lumen size. The stents acts as a scaffold to support the lumen in an open position. Various configurations of stents include a cylindrical tube defined by a mesh, interconnected stents or like segments. Some exemplary stents are disclosed in U.S. Pat. No. 5,292,331 to Boneau, U.S. Pat. No. 6,090,127 to Globerman, U.S. Pat. No. 5,133,732 to Wiktor, U.S. Pat. No. 4,739,762 to Palmaz and U.S. Pat. No. 5,421,955 to Lau. Balloon-expandable stents are mounted on a collapsed balloon at a diameter smaller than when the stents are deployed. Stents can also be self-expanding, growing to a final diameter when deployed without mechanical assistance from a balloon or like device.
Stents have been used with coatings to deliver drug or other therapy at the site of the stent. The coating can be applied as a liquid containing the drug or other therapeutic agent dispersed in a polymer/solvent matrix. The liquid coating then dries to a solid coating upon the stent The liquid coating can be applied by painting, spraying, dipping, wiping, electrostatic deposition, vapor deposition, epitaxial growth, combinations thereof, and other methods, including dipping or spraying the stent while spinning or shaking the stent to achieve a uniform coating. Combinations of the various application techniques can also be used.
The number of drugs suitable for use with stents in treating various pathologies in an artery or other body lumen is growing. New discoveries give rise to new drugs that may be effective in treating one or more pathologies present in a particular case. Although a combination of the drugs may be desirable to treat the different pathologies, the drugs, their preferred polymers, or the solvents required for application to a stent can be incompatible. The incompatibilities can both cause manufacturing problems and reduce the effectiveness of the therapeutic agents during use.
The compatibility problem can arise several ways in mixing more than one drug for application and use on a stent. First, the drugs themselves can be incompatible. Second, the drugs can have different solubility in a particular solvent, so that one drug dissolves easily, but the other drug is difficult to get into solution. In the extreme case, one drug may not be soluble in the preferred solvent for the other drug, so that two different solvents are required. In addition, the preferred polymer for one drug may be incompatible with the preferred polymer for the other drug. Needless to say, such factors can make the precise selection of materials difficult when two or more drugs are to be delivered. A uniform coating with different drugs contained in a single polymer can also limit the therapy options available. Although the preferred therapy may be to deliver one drug rapidly and another drug more slowly, both drugs are limited to their respective diffusion rates from the single polymer. In another case, it may be desirable to use a biodegradable polymer with one drug and a non-biodegradable polymer with another drug.
U.S. Pat. No. 5,383,928 to Scott et al. discloses a sheath for encompassing at least a portion of a stent to locally deliver a drug to an arterial wall or lumen into which the stent has been inserted, comprising a polymer and a drug incorporated within the polymer, the polymer sheath encompassing at least a portion of the stent and having a thickness to allow controlled release of the drug.
WIPO International Publication No. WO00/12147 to Yang et al. discloses a device adapted for mounting on a stent, the device comprising a sheath being made of polymeric material that includes drugs such as pharmaceutical agent(s) or radioactive agent(s) for delivery to an implant site. The sheath includes a main body of generally tubular shape, and may include mounting means for attaching same to stent. The device may have a slit therein, and may comprise a helical coil, a cylinder or any other suitable shape or design which fits a particular stent. The sheath may include a coating or coatings thereon containing drugs, surgical adhesives or a combination thereof.
It would be desirable to have a stent having an intermittent coating that would overcome the above disadvantages.